1. Field of the Invention
The present invention relates generally to the production of elastomeric compositions and, more particularly, to the mixing and production of rubber. Specifically, the present invention relates to an improved process for the continuous mixing and production of elastomeric compositions.
2. Description of the Prior Art
The conversion of elastomeric compositions, and in particular elastomeric polymers such as rubber, into commercially significant materials has been a focal point of the rubber industry throughout its history. The earliest commercial process was largely based on a mixing process utilizing two-roll mills to modify the basic polymer's characteristics by adjusting the viscosity of the polymer as well as by incorporating enhancing ingredients such as reinforcing agents, modifying oils and curatives. Over the years, a wide variety of mixing processes and devices have evolved in the rubber industry. A thorough treatment of such development is set forth in an article entitled, "Development of Internal-Mixer Technology for the Rubber Industry" by James L. White, Rubber Chemistry and Technology, Volume 65, page 528, the contents of which are specifically incorporated herein by reference.
The elastomer mixing process is basically defined as combining the ingredients specified in an elastomeric recipe, on some schedule of addition, and under some regime of mastications for a certain period of time and/or temperature. Rubber elastomer mixing technology is focused principally upon controlling five principal features of the mixing process independent of the recipe specifics. One feature of concern is the uniform distribution of raw materials of the elastomer recipe throughout a single unit mass or volume. The second feature involves dispersive mixing which focuses on the intimate association of the elastomer ingredients in a physical sense, typically at a molecular level, which influence the mechanical characteristics of the final composition. The third feature involves viscosity modification while the fourth feature is concerned with chemical reactions of the ingredients. The final feature involves, of course, economic considerations which control capital utilization and manufacturing costs. These five factors are interrelated, often inversely, and they are sensitive to rate, volume, power, time and temperature limitations.
With the advent of the Banbury mixer, which is described in the above-referenced article, and its commercial adoption, the rubber industry was able to substantially increase uniformity of the process and productivity of manufacturing elastomeric compositions. The Banbury-type intensive mixer, and its many modifications and adaptations as indicated, still remains the primary processing device and process in the rubber industry throughout the world today.
While the Banbury-type intensive mixing process has been adequate, it has certain limitations. Principal among such limitations is the batch or unit production nature of the process. This process requires a minimum of several passes through the Banbury mixer in order to incorporate all ingredients of an elastomeric composition since the Banbury is a batch-type process. This feature is the principal source of variation in characteristics of the final mixed elastomeric stock. Variation in the weights of individual ingredients, the order of addition, timing of addition and discharge, initial temperature of the raw materials and ingredients, and the process environment all contribute to batch-to-batch variation. Even with recent improvements to the Banbury-type process which include automatic weighing systems and computer controlled batch cycles, batch-to-batch variation control remains barely adequate requiring sophisticated protocols for batch blending in subsequent processes.
Another limitation of the Banbury-type process involves the volumetric dynamics of the large working volume of this process. Economic necessity dictates that Banbury-type intensive mixers be scaled to the largest size appropriate to the manufacturing operation. As a result, effective volumetric mixing of a high viscosity mass becomes even more difficult. Concurrent with limitations in volumetric mixing efficiency are thermodynamic control problems due to the marginal, and declining, surface area to mass ratio of increasing volumes within the Banbury mixer.
In order to avoid the aforementioned problems associated with batch mixing processes such as utilized by the Banbury mixer, the rubber industry has attempted to devise continuous mixing systems utilizing a variety of extrusion-type devices. The driving incentives for devising such continuing processes include improved uniformity through steady-state processing, better thermal management resulting from improved surface-to-mass ratios, and developing opportunities for highly automated operations. A variety of processes and devices have been created in attempts to incorporate technical rubber and elastomeric mixing features with extrusion processing systems utilized in other industries. Such extrusion-type devices have been used in the plastics and baking industries for some time. An example of this is illustrated in U.S. Pat. No. 5,158,725. While such devices and processes have enjoyed significant applications in the rapid expansion of thermoplastic polymer processing, none have been widely successful for adaptation by the rubber industry.
A combination of diverse forces have inhibited, up to now, the adoption of continuous extrusion-based processes by the rubber industry. Principal among these forces has been the consistent inability of extrusion-based processes to demonstrate improved uniformity at economically productive rates while maintaining the desired characteristics of the final elastomeric composition. Other factors have included sparse availability of suitable polymer forms, barely adequate mass flow metering systems, and continued inability to control the thermodynamics of the process. Such a lack of thermodynamic control has resulted in unacceptable blending and mixing and in premature curing of elastomers within the extrusion devices. The present invention overcomes the aforementioned disadvantages of batch-type Banbury mixers, as well as overcomes the disadvantages of prior continuous processes such as the control of process thermodynamics and uniform elastomer mixing.